PHYSICAL REVIEW B 83, 035314 (2011) I 2 basal plane stacking fault in GaN: Origin of the 3.32 eV luminescence band Ingo Tischer, * Martin Feneberg, Martin Schirra, Hady Yacoub, Rolf Sauer, and Klaus Thonke Institut f ¨ ur Quantenmaterie/Gruppe Halbleiterphysik, Universit¨ at Ulm, D-89069 Ulm, Germany Thomas Wunderer and Ferdinand Scholz Institut f ¨ ur Optoelektronik, Universit¨ at Ulm, D-89069 Ulm, Germany Levin Dieterle, Erich M¨ uller, and Dagmar Gerthsen Laboratorium f¨ ur Elektronenmikroskopie, Universit¨ at Karlsruhe, D-76128 Karlsruhe, Germany (Received 7 May 2010; revised manuscript received 23 November 2010; published 19 January 2011) We investigate the 3.32 eV defect-related emission band in GaN correlating transmission electron microscopy and spatially and spectrally resolved cathodoluminescence at low temperature. The band is unambiguously associated with basal plane stacking faults of type I 2 , which are a common defect type in semi- and nonpolar GaN grown on foreign substrates. We ascribe the luminescence to free-to-bound transitions. The suggested intrinsic acceptors involved have an ionization energy of ≈0.17 eV, and are located at the I 2 -type basal plane stacking faults. DOI: 10.1103/PhysRevB.83.035314 PACS number(s): 78.55.Cr, 71.20.Nr, 71.55.Eq, 71.70.Gm I. INTRODUCTION Standard commercial light emitting and laser diodes em- ploying GaN grown along the c direction suffer from the strong polarization built in along the 0001 direction, which reduces the efficiency of these emitters. Therefore it is of great interest to investigate GaN grown along orientations different from 0001. Heteroepitaxial growth of these semi- or nonpolar GaN often results in a large number of structural defects such as stacking faults. 1 The presence of stacking faults gives rise to characteristic luminescence features all in the region of 3.3−3.4 eV. Liu et al. 2 and Mei et al. 3 identified a band at 3.41 eV as originating from basal plane stacking faults (BSFs) (Ref. 4) of type I 1 . This interpretation was also supported by other groups. 5 , 6 Another emission band around 3.33 eV was assigned to stair-rod dislocations or prismatic stacking faults (PSFs) terminating BSFs. 2 A third band at 3.30 eV was found to be also related to PSFs. 3 Paskov et al. 5 reported a band at 3.322 eV, which was assigned to structural defects 7 and to basal plane stacking faults in regions where PSFs and partial dislocations are absent. 6 Alternatively, donor-acceptor transitions (DAPs) at the surface were suggested to be the origin of the band at 3.32 eV. 8 Whether these emission bands are identical albeit shifted from sample to sample by different strains cannot be decided easily. Eventually, all these bands have indeed different origins; this has to be clarified in future studies. In this study we focus on the band around 3.32 eV. Using a sample with well separated stacking faults, we are able to establish a direct correlation between basal plane stacking faults of type I 2 and this specific luminescence feature. II. EXPERIMENTAL DETAILS The sample under investigation consists of selectively over- grown GaN stripes with triangular cross sections. The stripes are aligned along the GaN 11 ¯ 20 direction (a direction), as defined by a photolithographically structured SiO 2 mask, and have semipolar {1 ¯ 101} side facets (Fig. 1). The whole structure is grown on c plane sapphire. Details of the growth process can be found elsewhere. 9 Compared to Ref. 9, an increased nonoptimal growth temperature was intentionally used creating a higher concentration of structural defects than the usual samples have. For a detailed investigation of the optical properties found in the triangular cross section, the sample was cleaved in order to have an a plane cross section. Another piece of the sample was mounted on a tilted sample holder to obtain a view on a {1 ¯ 101} side facet without having to deal with shadowing effects from the adjacent three-dimensional structures. The position on the sample surface in the latter case was marked, which allowed subsequent investigation of exactly the same position by transmission electron microscopy (TEM). The sample was first investigated by cathodolumines- cence (CL) performed in a scanning electron microscope (SEM) (Zeiss LEO DSM 982) modified so as to allow the insertion of a glass fiber for CL collection. The light is spectrally analyzed using a nitrogen cooled charge- coupled device camera mounted to a 90-cm focal length monochromator. Monochromatic CL images are recorded with a photomultiplier tube connected to a 25-cm focal length monochromator. The setup enables the investigation of luminescence with both high spatial (≈40 nm at 2 kV) and spectral (better than 0.1 meV at 3.5 eV) resolution. A helium cooled cryostat allows us to control the temperature. All CL measurements, except for the temperature series, have been performed below 10 K nominally. Electron acceleration voltages between 2 and 4 kV have been used for all CL measurements. A detailed structural characterization of exactly the same sample region as investigated by CL was performed subse- quently by conventional transmission electron microscopy and high-resolution scanning transmission electron microscopy (HRSTEM). A thin lamella was cut out from the GaN stripe (Fig. 1) using the focused ion-beam (FIB) technique 10 followed by argon-ion milling. A detailed analysis of the defect distribution was performed using a Phillips CM200 TEM system and an aberration-corrected Titan 3 80-300 transmission electron microscope. 035314-1 1098-0121/2011/83(3)/035314(6) ©2011 American Physical Society